1. Field of the Invention
This invention relates to a percussive mole for underground boring, such as for boring channels or passageways for underground utilities. More particularly, the invention relates to an electronic transmitter for surface detection of the location of the forward end of the mole.
2. Description of the Prior Art
Earth boring devices are known in the art and include both guided devices, for which the direction of forward progress of the mole can be controlled, and unguided devices. These devices are used for boring channels underground to allow for the installation of utility lines without necessitating the disturbance of surface obstacles, such as trees, fences, sidewalks and roads. For example, a hole may be bored beneath a road without closing the road and digging an open trench across it. The expense of rebuilding the road after installation of the utility is also avoided. If the obstacle is a building, an earth boring device allows for the installation of a utility line which would have been previously impossible.
A variety of boring rools are well-known for digging underground channels, including flexible rod devices, auger devices, pipe pushers, and air or hydraulic powered impact type piercing tools or percussive moles. The present invention is directed solely to percussive moles. These tools may or may not be guided or steerable once they enter the ground. Unguided, uncontrollable systems have a tendency to bury themselves, rise to the surface in the wrong position, or damage underground utility lines. Accordingly, they are used primarily for short bores of up to approximately 100 feet.
The forward or boring end of a percussive mole generally includes an anvil which is hit by an internal striker powered by compressed air. Generally, the rearward end of the mole is connected to a whip hose, which in turn is connected to a flexible air hose connected to a source of compressed air on the surface. One example of this type of mole is the PIERCE AIRROW.RTM. pneumatic underground piercing tool or mole. This percussive mole can also be adapted to both push or pull pipes through the ground.
Piercing tools or moles have been developed which provide both mechanical steering and orientation systems to overcome the problem of unguided devices. Typically a guided piercing tool consists of a slanted nose on a rotatable housing and an electronic instrumentation system for directional control. The slanted nose generates a deflective side force as the tool bores through the soil, thus permitting the operator to turn the tool in a desired direction. The means to appropriately rotate and control the tool are well-known and described in the literature.
FIG. 1 illustrates the general operation of a guided percussive mole earth boring tool as taught in commonly assigned U.S. Pat. No. 4,694,913 which is incorporated by reference. Drill rig 1 is disposed within launching pit 2 which is excavated to a depth below the level of desired horizontal bore hole 3 under a surface structure, for example, road 4. Drilling rig 1 is provided with an external source of compressed air 5 and is supported on tracks 6 within pit 2. The compressed air is linked to the drilling mole 7 which is supported at the forward end of hollow sectional drill rod 8. Drilling rig 1 supports drill rod 8 and permits the addition of further sections of rod as the drilling progresses through the earth.
Compressed air from compressed air source 5 is supplied through hollow drill rod 8 to pneumatic mole 7 which operates a hammer (not shown) to repeatedly contact an anvil member (not shown) connected to external boring element 9 having on angled cutting surface. Connector 10 is located between the rearward end of drilling mole 7 and includes a plurality of holes 11 for exhausting air from the drilling mole back into bore 3.
In order to avoid costly deviations from a desired path, it is important to know the position and direction of travel of a percussive mole at all times. This is important in both guided and in non-guidable percussive moles.
One solution known in the art to the problem of accurately determining the underground location of a mole is to use a transmitter (or sonde) attached to the mold. The transmitter transmits a signal to an above-ground receiver so that the location of the mole can be determined. However, because the transmitter must function in an extremely hostile environment of underground dirt and percussive boring, it is important to protect the transmitter as much as possible. For example, it is known to use a transmitter attached at the rear of the mole, such as to the whiphose linking the mole to the compressed air source. In this location the transmitter (or sonde) is relatively well protected from the high shock load on the mole body caused by the percussive impact. However, the exact location of the drilling bore element cannot be known with great accuracy, since the distance between the boring element at the front of the mole and the rear of the mole may be quite large, e.g., 3-6 feet. The mole would have to proceed for at least one body length before a detector located on the surface would detect that the mole was off-course. By this time it may have deviated to a large degree from the desired path and it may be too late to back the mole out of the bore to try a new bore, or in the case of a steerable boring device, correct the course of the mole back to its desired direction. Additionally, damage to sewers and utilities may have already occurred.
U.S. Pat. No. 3,746,106 (also incorporated by reference) shows a transmitter located in a housing between the boring bit and the bore pipe. The housing includes a "window", i.e., an area of the housing which allows transmission of a signal in the desired frequency range. The housing also includes a battery compartment and space for appropriate control circuitry. A rubber spacer is included in the battery compartment to continually urge the battery into contact with the terminal block.
Although the transmitter is located near the drill bit, the bit is designed to cut a hole through the earth by rotary action, progressively cutting the end face of the bore. Therefore, this design of the transmitter housing would be completely unacceptable in a percussive mole device since the impact on the mole creates shock forces which would quickly render the transmitter non-functional.